The combustion waste gases (i.e. the exhaust) of thermal power plants, factories, on-road vehicles, diesel generators, and the like typically contain SOx and NOx. State and federal regulations limit the permissible amounts of these emissions because they create environment problems, such as acid rain. Accordingly, there is a continual need for improvements in the cost effective and efficient control of these emissions.
One mechanism for limiting NOx and SOx emissions is to remove or scrub the pollutants from the exhaust gas using a sorption bed, trap or similar device. Because many NOx traps have been found to be poisoned by the presence of SOx, it is important to remove as much SOx from the exhaust gas as possible. However, as compared to the large volume of studies on NOx reduction, SOx removal using solid sorbents is an area in need of scientific advancement. Certain types of materials have been identified as possible solid sorbents for use in SOx sorption beds and traps, such as calcium oxide and alkalized alumina (Na/Al2O3 or K/Al2O3), copper-based sorbents (e.g., Cu/Al2O3), promoted metal oxides (e.g., TiO2, Al2O3 and ZrO2), promoted cerium oxide (La- or Cu-doped CeO2), and supported cobalt (Co/Al2O3). Unfortunately, over the temperature range of about 250° C. to 475° C., these materials typically have a relatively low sorption capacity. For example, their total sorption capacity of SO2 is typically less than about 10 wt % based on the weight of the sorbent, and their breakthrough sorption capacity can be substantially lower, depending on operating conditions. As it is combustion in this temperature range that leads to a significant portion of the total SOx emissions, a greater sorption capacity at these temperatures is needed.
One approach to increasing the sorption capacity of SOx sorption beds is to provide an oxidation catalyst upstream or admixed with the bed so as to convert most of the SO2 to SO3, since SO3 is generally more readily sorbed than SO2 due to its ability to form stable surface sulfates. However, the cost of recovery of the oxidation catalyst (frequently a precious metal) and the relatively poor conversion efficiency of SO2 to SO3 at temperatures below about 300° C. limits the effectiveness of this approach as well.